Measuring inside an active OLED

October 15, 2012 - Researchers at the National Institute of Advanced Industrial Science and Technology (AIST) and the Chemical Materials Evaluation and Research Base (CEREBA) say they have evaluated molecules within a sealed organic light-emitting diode (OLED) in operation using laser spectroscopy, measuring both selectively and nondestructively

Their work, published in August in Applied Physics Letters, involves a method improving upon a laser spectroscopic technique to measure molecular vibrations at the interface of an organic layer inside the OLED device — specifically, evaluating a signal enhancement phenomenon that occurs at the interface with a concentrated electric field.

The problem with evaluating OLED devices, as with many other types of sensitive electronics components: the method itself often involves destroying the device or impacting its performance (e.g. introducing contaminants). Measuring OLED device degradation, particularly in devices with multiple and overlapping internal layers, is particularly difficult — yet much more needs to be known about the inner workings of OLED layer degradation to learn how to extend the device’s lifetimes for application in displays or lighting.

Key to AIST’s work is using "sum frequency generation" (SFG) spectroscopy, which employs wavelength-tunable lasers to collect information on specific interfaces of organic substances in complex organic devices. Specifically it has pursued two-color SFG spectroscopy to measure vibrational changes at the surface and interfaces in a solid; one tunable visible laser would still collect signals from multiple organic lasers, but implementing two lasers creates a "double resonance" that can be used to enhance and isolate signals from a targeted organic layer. They also tweaked the SFG spectrometer to maintain measurement resolution even at 1/100 laser power of conventional SFG spectrometers.

"By investigating in detail the ‘fingerprints’ of organic layers in an OLED device, the alteration and degradation of molecules in the operating device as well as the change in the electric field inside the device can be elucidated," AIST explains. Their goal is to determine, at the molecular level, the driving mechanisms of OLED devices and their degradation — and also seek ways to apply the work to other organic electronics fields, such as solar cells and transistors.

CERERA was established at AIST specifically to establish design and manufacturing technologies for OLED materials and devices, including evaluation and analysis techniques.

Top: Schematic drawing of the structure of the multilayered OLED device and the directions of the incident and emitted lights used for SFG spectroscopy. Bottom: Spectral changes in an operating multilayered OLED device, with +8 V application (light emission), no voltage application, and –5 V application. (Source: AIST)

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